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Poor water quality can sometimes be the cause of fin rot. Gold fish also produce a lot of waste. As a rule of thumb you should be doing a minimum of 50% water changes and gravel vacs weekly.

Regardless, adequate filtration (you should have filters rated for at least 60 gallons on a 30 gal tank) water changes are the best cure for fin rot. Lots or them and big ones. I'd say 50% or more daily until the problem is cleared up. I have also used Melifix and Pimafix on neons and that has worked well in conjunction with the water changes.

Your gold fish will likely need a larger tank as they grow huge if kept in a proper sized tank. Depending on his size now, the small tank might might be part of the reason he's developed fin rot.

Good luck. I hope you get it cleared up. start doing those water changes :o)

adding salt to the water will also help speed up the healing, in conjunction with water changes.

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good point. make sure it's uniodized salt - canning salt will do if you don't have aquarium salt. Sea salt for human consumption will also work. no more than 1 tablespoon per 5 gallons. I think I'd even do half of that to start. Dissolve the salt in water drawn from the tank. Make sure it's well dissolved, then add it in increments throughout the day so as not to shock the fish. when you do a water change, replace the salt that you removed being careful not to over do it.

I agree totally with the salt treatment. I dealt with this just last week, when my black neon tetras got Ick and pretty much on the same day, my cory cat got bad fin rot on his tail fin, it was all jagged and ripped looking. I treated the tank for Ick without paying any attention to the fin rot really. I heated the tank to 86 degrees and added about 35 teaspoons of de iodized salt over 2 days. Like fishmommie said, I first dissolved the salt in tank water in a cup, then added it. I had lots of surface agitation from my 2HOB's, an aquaclear 50 and an aqueon 30. I then just let the tank sit for a week. About 5 days in I saw no more signs of Ick on my fish. Just yesterday I did a couple big water changes in order to get rid of the salinity, and I noticed my Cory's tail fin was back to normal. It was as if the fin rot had never occurred. Maybe it's just a coincidence, but I think the salt treated both ick and fin rot.

Salt definitely helps with goldfish, no doubt. But one of the best medications is good water quality. The fin rot bacteria is always there, but the fish's immune system can usually hold it off. However, poor water quality, stress, fin-nipping, or parasite/fungal/bacteria problems will weaken the fish's immune system, making it more vulnerable to problems like fin rot. Usually, if you solve the major problem, the fin rot will go away on its own.
More than likely, you just have a problem with water quality. Do partial water changes and add a bit of salt, and his fins should be back to normal in no time!